For those having the same problem, I offer a couple of other things you can check. First note that on the 500c/m an A-12 partially used properly loaded magazine will show part red/white in the window on the mag. If you remove the mag and wind the body if it's cocked it'll show white. If you can't get that indicator, you've likely got a problem with the camera body winding mechanism.

If your magazine isn't properly loaded and the take-up spool is just freewheeling OR if the film is loaded backwards, the frame counter will not show you it's advanced to frame #1 but will remain blank. Remember the take up spool is in the top of the mag, the film is loaded in the bottom and reversed to pull over the front of the mag and under the edge retention flap. OR remove the film holder after you fully open the locking mechanism on the mag. That allows the film counter to reset and then reload it without film, hang it on the back and remove the dark slide and see what happens.

Also check your serial numbers on the magazine. While all A-12 mags and inserts look alike they aren't. The 3 numbers on the film insert itself should match the last 3 digits of the mag film holder. These are matched at the factory. Mis-matched film inserts can cause minor difficulties in advancing frames and sometimes dealers accidentally switch them.

You can manually cock the shutter by opening the two curtains at the rear of the body, finding the larger slot at the bottom front of the body and taking a long screwdriver, turn that slot to the right until the red on the slot aligns with the red dot on the body.

Also double check to make sure your mirror isn't locked up.
M.